Four weeks after giving birth to daughter Maxwell (with fiance Eric Johnson, 32), Jessica Simpson, 31, has already curbed her high-calorie habits -- and not just because of that reported $4 million Weight Watchers deal.

"She's breast-feeding Max, and wants her to get the best nutrition," a source tells the new issue of Us Weekly (out Friday) of the singer, whose pregnancy snacks included Pop-Tarts and Kraft mac 'n' cheese. "She has to cut down on fast food. Besides, it's a great way to jump-start the massive diet ahead."

Last November, Us Weekly broke the news that the 5-foot-3 star was in negotiations to work with Weight Watchers to get her pre-baby body back. On Wednesday, Simpson took to Twitter to confirm the deal: "So excited to be part of the Weight Watchers family," she tweeted.

Later that day, she further explained her decision to give the weight-oss program a try on her Facebook page. "The cool thing about [Weight Watchers] is that it focuses on healthy habits for the long-term (and I can still indulge in my guilty pleasures every now and then, too)," Simpson wrote. "I have actually gotten a group of friends together who are going to be doing it with me."